ANPR/ALPR Case Study
by Aoife Chapman, Technical Product Manager - ProPhotonix Limited Posted 07/03/2012
Company: NDI Recognition Systems
Industry: ALPR/ANPR Systems
Product: SpecBright Spotlight
Description: Infrared (IR) Illuminators for ALPR/ANPR Systems
NDI Recognition Systems (NDI-RS) is a leading US and UK-based designer and manufacturer of Automatic License/Number Plate Recognition (ANPR) systems for the high technology security, surveillance and traffic management markets.
Requirement
Illumination device capable of supporting mobile systems affixed to patrol cars, portable covert systems and static or fixed ALPR/ANPR systems. The challenging attributes of the illuminator were: range of wavelengths; intensity; size; strobe; and durability.
Solution
ProPhotonix customized LED SpecBright Spotlight
The ALPR/ANPR market requires systems that operate in demanding environmental conditions, with variable lighting (day, night, inclement weather) and, when mobile, having the ability to withstand vibration/movement.
As part of a continuing research and development program, NDI-RS selected ProPhotonix to provide an improved illuminator for integration into their systems. The end-product is for two separate models: The first is a system suitable for both stationary and mobile installations. The second a more compact system designed primarily for mobile, vehicle-based deployment
NDI-RS needed a compact, high intensity, strobable IR light source in a range of wavelengths to suit differing customer requirements. Lifetime was also a key consideration. Prophotonix was chosen for its custom engineering capabilities, extensive LED expertise, and full solution service. Each challenge is addressed to fulfil NDI-RS requirements.
Wavelength
Infrared light is the best suited for most ALPR/ANPR systems. Vehicle number plates are often made with retro-reflective materials designed to reflect IR radiation. An added benefit of IR illumination is that it is invisible to the human eye, won't distract drivers and is ideal for covert surveillance. NDI-RS operates globally and must offer their systems in a range of IR wavelengths to accommodate the variability from one jurisdiction to another.
For most European ANPR systems, 870 nanometer (nm) is the most suitable wavelength. However, in the United States, where license plate regulations vary by region, alternative IR wavelengths e.g. 940 nm, are sometimes found to be more suitable for use. ProPhotonix developed solutions for NDI-RS to provide output wavelengths of 810nm, 870nm and 940nm.
Intensity
To achieve established accuracy and reliability standards, NDI -RS required illumination components to provide adequate illumination over a 15-25 meter (m) working range.
To maximize source intensity, ProPhotonix designed a chip-on-board (COB) array consisting of tightly spaced, high intensity LEDs and beam shaping. System efficiency is maximized by incorporating beam collimation design and a total internal reflection (TIR) lens which together ensure that photons that exit the LEDs at high angles are reflected towards the target providing an intense beam at longer working distances.
Size
The required solution is used in both the mobile and stationary ANPR systems. Mobile units require a compact mechanical size of illuminator, confined by the small size of the system. In addition, as IR light is retro-reflected by target number plates, the gain factor of the system camera is maximized by aligning the camera and IR source as closely as possible, further restricting the mechanical design of the solution.
ProPhotonix utilized the compact mechanical size of the SpecBright Spotlight for easy integration into both NDI-RS products allowing excellent source alignment with the system cameras. A compact design was accomplished using chip-on-board (COB) technology which allows the development of products from the chip up.
Strobe Capability
ALPR/ANPR systems send a signal to the illumination source to provide a series of IR strobe pulses which synchronize with the image capture by the system camera. Typically, to accommodate a variety of environmental conditions, both the duration and intensity of the IR pulse is varied by the ANPR system. Strobe capability was a key requirement of the project.
ProPhotonix utilised the Specbright Spotlight which is suitable for continuous use at an operating current of 400 mA. In order to achieve the required intensity a short pulse with a low duty cycle was applied through a unique circuit to both modulate and overdrive the LEDs. This obtained the high output strobe illumination at a distance up to 25m.
Durability
ANPR systems are designed to operate continuously (24/7) in all weather and where appropriate in mobile situations. NDI-RS required long lifetime of the illumination component. ProPhotonix considered all of NDI-RS requirements in the selection of LEDs to meet their needs. After considering NDI-RS parameters, the optimum LEDs were selected and the SpecBright Spotlight has a lifetime of up to 100,000 hours.
The Specbright Spotlights developed for this project, are strobable 810 nm, 870 nm and 940 nm sources, capable of providing high quality illumination over a 15-25 m range. The Specbright has been successfully incorporated into the NDI C320 and V220 camera systems, which are currently utilized in a large number of applications in the security sector.
These include the London, Manchester and Brighton 'Ring of Steel' security and surveillance cordons, which monitor vehicle entry and exit, to and from, a defined inner city limit. NDI-RS ANPR systems have also been used by numerous police forces worldwide, to conduct surveillance of strategic motorways, bridges and tunnels.
"Components like ProPhotonix's SpecBright spotlights were incorporated into the C3 system as they are high quality and allow the cameras to achieve an accurate read and meet guidelines."
NDI Recognition Systems
